Work vehicles such as tractors and backhoes, for example, tow implements using a variety of hitches to connect the vehicle and the implement. One of the most common hitches is called a "three point hitch." It is called a three point hitch since it has three substantially parallel links or connections that extend between the vehicle (usually a tractor) and the implement. These links permit the motion between the vehicle and the implement to be constrained to relative translation, and substantially prevent relative rotation.
Many tractors or other work vehicles are able to electronically monitor and control their travel through the field, as well as control the height of and load placed upon the implement. This control is especially useful to regulate the speed of the tractor, the power output of the engine, the tractor's gear ratio and the height of the implement above or below the ground. Using various sensors on the vehicle, the tractor can raise the implement to reduce the implement load, increase the power output of the engine, down shift to a lower gear or reduce the speed of the tractor.
To sense the load placed on the tractor by the implement, a load sensor is typically provided between the implement and the tractor to sense the load placed on one or more of the links connecting the tractor to the implement. These sensors can be arranged to provide two different types of sensings: indirect and direct load sensing. Both of these methods have serious limitations. The direct method places a load sensor between the tractor and the implement that directly senses the strain applied by the implement on the tractor. As an example, a load pin is placed between one or more of the three links and the tractor or between one or more of the three links and the implement. A common direct sensing method has been to replace one or more of the pins coupling the three links of the three point hitch to the tractor or one or more of the three links connecting the three point hitch to the implement with a load sensing pin that provides an electronic signal indicative of the load applied to the pin. As the links are pulled by the tractor towing the implement, they in turn apply a force to the load pin. Load cells placed on the load pin provide an electrical signal indicative of the load on the load pin.
There are problems with the direct method. In the direct method, the pin is fixed with respect to the tractor. A bracket fixed to the tractor commonly has two ears or webs with holes through which the load pin extends. The bracket defines the axis of the load pin perpendicular to the direction of travel of the tractor, extending either vertically or horizontally, and prevents the pin from moving. Since the load sensed by the load pin typically indicates a load applied perpendicular to the load pin axis, it is responsive only to loads applied straight backward away from or forward toward the tractor. As the implement moves from side to side, however, it places a bending or torsional load on the load pin. This causes the load pin to give a false reading of the load in the link.
In the indirect method a spring member is located between the upper link and the tractor. As a load is applied to the upper link, the spring member deflects. This deflection is either sensed by a position sensor or a load sensor. While this eliminates some of the problems of the direct method, it is less accurate than the direct method.
What is needed, therefore, is an improved method and apparatus for sensing the load applied by an implement to a tractor. It is an object of this invention to provide such a method and apparatus.